The invention relates to a picture display device comprising a video processing stage for supplying an incoming video signal containing at least a composite synchronizing signal, a synchronizing signal separating stage connected to the video processing stage for supplying the composite synchronizing signal, a line synchronizing circuit coupled to the synchronizing signal separating stage for generating a line reference signal which is substantially synchronous with the line synchronizing signal present in the composite synchronizing signal, and a noise detector coupled to the video processing stage for detecting noise in the incoming video signal and for changing-over portions of the picture display device in dependence on the detected noise signal, this noise detector comprising a noise separating stage for differentiating the incoming video signal and for amplifying the resultant noise signal during the occurrence of a line signal, and also an output stage for supplying a change-signal derived from the amplified noise signal.
Such a picture display device is disclosed in European Patent Application No. 59,379. In the television receiver described therein a number of portions of the receiver are changed so as to improve the displayed picture when a large amount of noise is received. To that end, the incoming video signal is delayed before it is applied to the synchronizing signal separating stage. The line synchronizing signal is generated with the aid of the delayed signal. During the occurrence of the line synchronizing signal, the noise obtained by differentiation is amplified and thereafter, the amplified noise signal is subjected to half-wave rectification. This results in a d.c. voltage. The value of this voltage depends on the quantity of noise contained in the video signal. When a predetermined value is exceeded, this voltage provides the change. The differentiation of the video signal produces disturbing pulses during the occurrence of the edges of the non-delayed line synchronizing pulses. Thanks to the delay, the disturbing pulse during the occurrence of the leading edge is removed, whereas the disturbing pulse caused by the trailing edge can be removed by half-wave rectification.
If a great deal of noise is present then it may, happen that the synchronizng signal separating stage in the prior art arrangement is not capable of producing an adequate line signal for the noise separating stage. The weak line signal obtained may be shifted in time relative to the incoming video signal, for example, due to reflections, so that no proper distinction can be made between the noise and the higher components of the video spectrum, i.e. luminance and, more specifically, chrominance components, with the result that the noise detector does not supply reliable information. A large-amplitude disturbing pulse occurring during the line trace period may also produce a signal for the noise separating stage.